1. Field of the Invention
This invention relates to the conversion of radio frequency (RF) signals.
2. Background Information
In general, wireless communications comprises the modulation of one or more baseband information signals onto one or more carrier signals, transmission of the resulting bandpass signal(s), and demodulation at a receiver to recover one or more of the information signals. Modern receivers typically employ the heterodyne technique, which involves either down-converting or up-converting an input RF signal to some convenient intermediate frequency (IF) and then demodulating the IF signal by using an appropriate detector. Heterodyne receivers are easily tunable and offer high stability. The difference between the input and output frequencies of such a receiver also provides a high degree of immunity from self-oscillation due to stray coupling. Additionally, adjacent channel rejection may be obtained by using high-Q filters only in the IF stage, which may operate at a fixed frequency much lower than the carrier frequency.
A basic heterodyne conversion circuit as shown in FIG. 1 may be used to convert all types of modulated RF signals to IF, including broadcast-band AM, FM and television signals; network communication signals as in a cellular telephone or wireless local area network; satellite communications or ranging signals; and radar signals. In such a circuit, the mixer receives the RF signal S10 (for example, as outputted from a RF amplifier) and multiplies it with a signal S20 from a local oscillator 5 to produce an IF signal.
We define the carrier frequency of RF signal S10 to be ωc, the frequency of local oscillator signal S20 to be ωLO, and the desired frequency of the IF signal to be ωIF (all in radians/second). Therefore, we may express RF signal S10 as cos ωct, local oscillator signal S20 as cos ωLOt, and the desired IF signal as ωit (with t in seconds). With reference to the trigonometric identitycos a cos b=(½)[ cos(a+b)+cos(a−b)],we can see that the output of the mixer will include a downconverted signal cos (ωLO−ωc)t and an upconverted signal cos (ωLO+ωc)t. The IF filter is a bandpass filter that receives the output of the mixer and selects either the up-conversion result or the down-conversion result, whichever is chosen by the receiver designer.
FIGS. 2A and 2B are graphical illustrations of heterodyne conversion operations using low-side injection and high-side injection, respectively. In these operations, we assume that downconversion is desired [i.e. ωIF=|(ωLO−ωc)|]. Now consider a case in which RF signal S10 contains not only the desired component at ωc, but also an undesired image component at a frequency ωi=2ωLO−ωc. In both examples, the image component will also downconvert to corrupt the desired IF signal at ωIF. These figures illustrate a major weakness of the basic heterodyne design: its susceptibility to image interference. In order to prevent such a situation, heterodyne designs usually include an image reject filter upstream of the mixer (e.g. as shown in FIG. 3) in order to attenuate any image components before mixing.
Unfortunately, the need for an image reject filter may greatly increase the size and cost of devices such as wireless communication apparatus. Depending on the design requirements of the filter, it may be physically large and very expensive. A need to implement the filter at RF frequencies rather than IF frequencies may compound the difficulty of obtaining a component that is suitable in terms of cost, size, and performance. Additionally, such a filter will typically be supplied as an off-chip component, thereby increasing fabrication costs, necessitating extra pins on the RF/IF chip, and consuming board space. Such requirements are contrary to the increasing need to reduce the size and cost of wireless communications devices, especially in the field of cellular telephony.
FIG. 4 shows a block diagram of a Hartley image reject mixer 100. Such a mixer may be used in a heterodyne conversion circuit (e.g. as shown in FIG. 5) as a smaller and less expensive alternative to an image reject filter. Unfortunately, the rejection performance of this approach is highly dependent on very close matching between the two signal paths in terms of both gain and phase. Moreover, even under careful manufacturing conditions, such an image reject mixer achieves good results only over a limited frequency band. Shortcomings such as these make the configuration of FIG. 5 unsuitable for applications that require high levels of image rejection (e.g. greater than 35–40 dB).